The Role of Neuroinflammation in Regulating the Age-Related Decline in Neurogenesis

Adult neurogenesis, is a lifelong process by which relatively few cells are added into two restricted regions of the brain. Integration of the cells into the existing neuronal circulatory, with the unique properties involved in the maturation of these cells, is possibly critical to the acquisition and retrieval of new memories. With the chronological aging of the organism a process of cellular senescence occurs throughout the body; a portion of which is independent of primary alterations to the stem cells; instead, it appears to be dependent on the environment where the cells reside, and is in part regulated by inflammation. Microglia, the resident immune cells in the brain, are neuroprotective but chronic activation of the microglia, such as the chronic activation that occurs with advanced age, can promote neurotoxic inflammation. However, it not clear if the aged-related increase in neuroinflammation is at least partly responsible for the aged related decrease in neurogenesis. To address the involvement in neuroinflammation in regulating neurogenesis we used 3 different potential therapeutically relevant manipulations. The first was a targeted approach directed at disrupting the synthesis of Interleukin-1beta (IL-1B), which is a proinflammatory cytokine that is consistently found elevated in the aged brain. The second was a cell therapy approach in which human umbilical cord blood cells were injected into the systemic circulation. The final approach was directed at a chemokine system, fractalkine/CX3CR1, which has been shown as an important paracrine signal, from neurons that regulates the activation state of microglia. While the three approaches used to manipulate, aging-rodent model system were different, a consistent finding was reached in all three studies. In the aged brain, microglia which are the predominate produces of IL-1B, negatively regulate neurogenesis. When IL-1B is decreased or microglia activation is decreased, neurogenesis can be partially restored in the aged brain. The results of these studies, demonstrate a key role for microglia in regulating the neurogenic neiche, which are amendable to therapeutic manipulations